Well, the developer already did, but the image pack contains images from a release version that's originally behind a paywall (requires a $20 tier subscription, if I remember correctly). It's "just" bonus contents for supporters.
If you've played the SE version before, and the save file you're using is from that, then your save is expecting to play the Patreon release, while the release currently available in OP is the standard release. It's basically a flag set in your save file, so to avoid issues/errors, you either have to start over with a new save, add the third party image pack, or find a way to set the "Patreon release" flag in your save to not see the bonus contents. For the latter, the most user-friendly way is to install KoGa3's mod, and change the "Patreon contents" to false via the mod settings.